Reflective optical elements for the extreme ultraviolet (EUV) and soft x-ray wavelength range (e.g., wavelengths between approximately 1 nm and 20 nm) such as photomasks or multilayer mirrors, are used in particular in the lithography of semiconductor components. Because EUV lithography devices typically have multiple reflective optical elements, they must have the highest possible reflectivity to ensure a sufficiently high total reflectivity. Because typically multiple reflective optical elements are situated one behind another in an EUV lithography device, even small reflectivity losses at each individual reflective optical element have a greater effect on the total reflectivity.
Reflective optical elements for the EUV and soft x-ray wavelength range typically have multilayer systems. These are alternatingly applied layers of a material having a higher real part of the index of refraction at the operating wavelength (also called a spacer) and a material having a lower real part of the index of refraction at the operating wavelength (also called an absorber), an absorber-spacer pair forming a stack. A crystal is thus simulated in a certain way, whose lattice planes correspond to the absorber layers at which Bragg reflection occurs. The thicknesses of the individual layers and also of the repeating stack may be constant or also vary over the entire multilayer system, depending on which reflection profile is to be achieved.
One approach for ensuring the highest possible total reflectivities in EUV lithography devices is to provide reflective optical elements with a protective layer of one or more layers, to protect the reflective optical elements from contamination. The service life of the reflective optical elements is thus lengthened and a reflectivity loss over time is reduced.
A further approach supplements the basic structure made of absorber and spacer with further more and less absorbent materials to increase the maximum possible reflectivity at the particular operating wavelength. For this purpose, in many stacks, absorber and/or spacer materials may be exchanged with one another or the stack may be constructed from more than one absorber and/or spacer material. The absorber and spacer materials may have constant or also varying thicknesses over all stacks to optimize the reflectivity. In practice, higher reflectivities cannot be achieved to the expected extent in reflective optical elements having complex material sequences.